Critical Factors Affecting the Catalytic Activity of Redox Mediators on Li–O2 Battery Discharge

Redox mediators (RMs) have a substantial ability to govern oxygen reduction reaction (ORR) in Li–O2 batteries, which can realize large capacity and high-rate capability. However, studies on understanding RM-assisted ORR mechanisms are still in their infancy. Herein, a quinone-based molecule, vitamin K1 (VK1), is first used as the ORR RM for Li–O2 batteries, together with 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ), to elucidate key factors on the catalytic activity of RMs. By combining experiments and first-principle computations, we demonstrate that the reduced VK1 has strong oxygen affinity and can effectively retard the deposition of Li2O2 films on the electrode surface, thereby guaranteeing enough active sites for electron transfer. Besides, the low reaction free energy of disproportionation of the Li(VK1)O2 intermediate into Li2O2 also significantly accelerates the ORR process. Consequently, the catalytic activity of VK1 is significantly boosted, and the discharge capacity of VK1-assisted batteries is 3.2–4.5 times that of DBBQ-assisted batteries. This study provides new insight for better understanding the working roles of RMs in Li–O2 batteries.